We talk a lot about basal body temperature (BBT), which is your body's temperature at rest. Taking your BBT when you wake up in the morning is an easy way to see when you’re ovulating, or whether you already have. If you’re trying to conceive, confirming that you’re ovulating rules out anovulation (not ovulating) as a potential cause of infertility. If you’re trying to avoid pregnancy, the temperature shift that confirms ovulation gives you the greenlight (after a few more days) that it’s safe to have unprotected sex.
The shift in your BBT is usually between 0.5 degree and 1 degree Fahrenheit (1), and an array of factors in your life may mask the subtle shift. These factors influence the accuracy of your BBT and are important to know. Here, we outline why BBT increases, certain circumstances to watch for when tracking BBT, and easy hacks to keep your BBT readings on track.
How do I take my BBT?
There are a number of guidelines for taking an accurate BBT reading, but the chief one boils down to this: take your temperature around the same time after waking and as soon as you wake up. Take it before getting out of bed, getting a drink, taking a sip of water, or eating anything. Don’t pet your dog, don’t smooch your partner. Take your BBT first. Doing anything before taking your BBT in the morning could affect your temperature.
When does BBT rise?
BBT is slightly lower in the first half of your menstrual cycle, which is the follicular phase. Then it rises after you ovulate due to a surge of progesterone, which readies the uterus for egg implantation. Once this happens, BBT stays slightly elevated through the second half of your cycle, the luteal phase. This phase is pretty consistent in most women, usually lasting 14 days (2), though some variation in luteal phase is normal. (3)
What makes BBT more accurate and less accurate?
There are a number of guidelines for taking an accurate BBT reading, but the chief one boils down to this: take your temperature every morning — as soon as you wake up. Take it before getting out of bed, taking a sip of water, or eating anything. Don’t pet your dog, don’t smooch your partner. Take your BBT first. Doing anything before taking your BBT first thing in the morning affects your temperature.
That said, certain factors may throw off your BBT reading. Here are some of the common culprits, along with pro tips to deal with these circumstances.
A poor night’s sleep
You need at least three consecutive hours of sleep for an accurate BBT reading. Tossing and turning all night will likely affect your temperature. While everyone has a crummy night of sleep now and then, if prolonged sleeplessness is a problem, try switching up your nighttime routine. Read a physical book (no bright screens!) before bed or listen to a calming app to improve your ZZZZZs.
BBT should be measured within about one hour of the same time each day because, for some, temperatures may creep up the later it’s measured. Some find that they can get up and go to the bathroom before temping, but this takes several cycles plus some trial and error to figure out (TCoYF p89). When you’re first starting out, being consistent and temping immediately before doing anything else is the best practice.
Drinking alcohol the night before
Enjoying an alcoholic beverage at night can affect your temperature. Still, if a glass of wine helps you unwind, here’s a hack that might help. Track your temps for a few weeks, making a note on the mornings after you’ve had a drink or two. If your BBT looks consistent across a few cycles, then you know alcohol (or, at least, the specific type and number of drinks you had) isn’t affecting your BBT.
Hot or cold rooms
Sure, it’s nice to crank up the heat or crawl under an electric blanket on a chilly night, but the toasty environment may affect your temperature. This isn’t true for everyone, but pay close attention to your BBT after sleeping in a room that’s warmer or colder than usual. If your temperatures are higher or lower than normal, you can mark that temperature as questionable. If not, then it’s safe to log your BBT per usual.
Travel can stress out your body and affect your temperature, which might be especially susceptible if you’re in a different timezone. Here’s what you can do: the morning after you arrive, record your BBT at your usual time. If you wake up at 6 a.m. at home, wake up at 6 a.m. in the new locale. Try to make everything as close to normal as possible. If your BBT looks consistent with the rest of your cycle, you can log that temp like you would at home. If not, travel is acting as a stressor on your body. Bear in mind it also may affect your ovulation.
Other factors that can affect your BBT include getting an infection, coming down with the flu or some other illness, or taking certain medications.
How do I solve BBT accuracy if I’m trying to avoid pregnancy?
If you’ve had to mark a series of days as questionable due to any of these circumstances, don’t assume you’re in your infertile phase until you see a clear pattern of high temperatures after ovulation. When in doubt, it’s always smart to use protection or abstain until your next cycle. If one of these circumstances happens after ovulation when you’re in the luteal phase, you don’t need to do anything differently, since you’ve already ovulated and are in the clear (as long as the waiting period has passed).
How do I solve BBT accuracy if I’m trying to conceive?
If these factors occur and you need to mark one or more temperatures as questionable during your fertile window, be sure to pay close attention to other clues, such as your cervical mucus and cervical position during that time.
Knowledge is power — the more data you gather about your body under different circumstances, the more you’ll understand your own temperature changes. When in doubt, mark your temperature that day as questionable. It’s always smart to investigate clues besides your BBT (cervical mucus, cervical position) to get the full picture of what your body is telling you, rather than focusing on a single temperature or one day in particular.